1. Field of the Invention
This invention relates to residual current devices.
2. Prior Art
Residual current devices (RCDs) detect earth fault currents, which are also known as residual currents. The principle of operation of RCDs is very well known.
RCDs can be grouped into two broad categories—voltage independent (VI) and voltage dependent (VD) types. The VI types use the detected residual current as the source of energy to enable them to operate. The VD types use the mains supply as the source of energy to enable them to operate. The VI types are commonly referred to as electromechanical types, and VD RCDs are often referred to as electronic types.
RCDs can also be divided into two other categories—those that are mechanically or magnetically latched (ML) to enable them to be closed and remain closed (these include VD and VI types), and those that are electrically latched (EL) in that they require the mains supply to enable them to be closed and to remain closed.
The EL types can be further subdivided into those that open automatically on loss of supply and remain open on restoration of the supply (for convenience here referred to as ELRO—electrically latched remains open), and those that open automatically on loss of supply but reclose automatically on restoration of the supply (for convenience here referred to as ELAR—electrically latched auto recloses).
These four RCD types are summarised as follow
VIML voltage independent mechanically latching
VDML voltage dependent mechanically latching
VD-ELRO voltage dependent electrically latching—remains open
VD-ELAR voltage dependent electrically latching—auto recloses
All of these RCDs have advantages and disadvantages which users can take into consideration when selecting an RCD for a particular application. For example, the VI type can operate down to virtually zero volts, but can be disabled under a double grounded neutral fault condition. The VDML uses electronic circuitry to provide for enhanced performance such as detection of pulsating DC fault currents, but can be disabled in the event of loss of supply neutral when used on a single phase supply. It should be noted that the risks involved in both of the above cases are generally considered to be very low and have not precluded the extensive use of such RCDs worldwide. The ELRO also uses electronic circuitry to provide for enhanced performance, and can protect motors from burn out under conditions of low supply voltage by auto tripping under these conditions. However, ELRO devices have to be manually reclosed on restoration of the supply, which can be an inconvenience. The ELAR uses electronic circuitry to provide for enhanced performance, and can protect motors from burn out under conditions of low supply voltage by auto tripping under these conditions, and also auto recloses on restoration of supply.
From the above, the ELAR would appear to be the ideal RCD. Unfortunately, the need to enable such RCDs to open automatically on loss of supply and reclose automatically on restoration of the supply has to date required very sophisticated and bulky electronic circuitry and components to provide this functionality. These problems are seriously compounded by the additional requirement for these RCDs to remain open after opening in response to a residual current, even if the mains supply is temporarily removed and restored after such opening.
FIG. 1 is an example of a simple ELAR RCD. In FIG. 1, an AC mains supply is fed to a load L via two contacts SW1 of a relay RLA, the live and neutral mains conductors L, N passing through a current transformer CT en route to the load. The output of the CT is fed to an RCD integrated circuit (IC) 10. The function of the CT and IC 10 is to detect a current imbalance in the AC supply to the load indicative of a residual current, and when such an imbalance is detected to provide a high output voltage on the line 12 sufficient to turn on a silicon controlled rectifier SCR1. The construction and operation of such components are well known. The IC 10 may be a type WA050, supplied by Western Automation Research & Development and described in U.S. Pat. No. 7,068,047. The RCD is powered from the mains via a bridge rectifier X1. The IC 10 is supplied with current via resistor R2.
A solenoid SOL, a capacitor C1 and the relay RLA are connected in parallel to the bridge rectifier X1 via a resistor R1. The SCR1, which is normally held in a non-conducting state by a low voltage on the line 12 from the IC 10, is connected in series with the solenoid SOL. The relay contacts SW1 are normally open. An example of a suitable relay RLA is shown in FIG. 2.
RLA comprises a bobbin 14 with a coil (not shown) wound on it. A ferromagnetic pole piece 16 extends through the bobbin, the top of the pole piece being positioned below a ferromagnetic element 18 fixed within a moving contact carrier 20. The contact carrier 20 and the moving contacts 22 are biased towards an open position away from fixed contacts 24 by a spring 26, so that a substantial air gap exists between the pole piece 16 and the ferromagnetic element 18. The coil has a relatively large number of turns in its winding so as to maximise the ampere turns providing electromagnetic energy.
Prior to the application of power to the RCD, the relay RLA is de-energised and its contacts 22, 24 are open, so the mains supply is disconnected from the load L (the moving and fixed contacts 22, 24 constitute the contacts SW1 of FIG. 1). When power is applied to the RCD the current through R1 will initially flow primarily to C1 to charge it up. As C1 acquires charge, more of the R1 current will be diverted into the RLA coil to establish an electromagnetic field which will be concentrated in the pole piece 16 and thereby provide an attracting force on the ferromagnetic element 18. At a certain threshold of ampere-turns known as the closing ampere turns the bias of the spring 26 will be overcome by the magnetic attraction between the pole piece 16 and the ferromagnetic element 18 and the contact carrier 20 will move automatically towards the pole piece 16 with the result that the contacts SW1 will close and thereby provide power to the load. Thereafter, C1 will sustain the RLA coil with current during the low voltage troughs of the rectified mains supply. A Zener diode ZD1 clamps the voltage on capacitor C1 to a safe level for the capacitor and the relay coil.
In the event of a sufficient reduction in the level of the mains supply voltage, the current through the RLA coil will be insufficient to keep the contact carrier 20 in the closed position, and the contact carrier and the contacts SW1 will automatically revert to the open position. If the mains voltage then increases to a sufficiently high level so as to reach or exceed the closing ampere turns current level, the relay RLA will be re-energised and its contacts SW1 will reclose automatically as before.
The RLA coil will have a relatively large number of turns to enable it to achieve the required number of ampere turns to cause automatic closing of the contacts SW1. This results in the RLA coil having a relatively high impedance, typically a few thousand ohms. In contrast the solenoid SOL will have a relatively low impedance, typically less than 200 ohms, because it will only be energised momentarily as will be described later.
In the event of a residual current of sufficient magnitude, the output of the CT will cause the IC 10 to turn on SCR1 via output line 12, which in turn will cause the solenoid SOL to be energised and open associated solenoid contacts SW2. This in turn will result in removal of supply to C1 and also cause C1 to discharge through the relatively low impedance of the solenoid SOL. The resultant discharge of C1 will cause the relay RLA to de-energise and its contacts to open. It is generally a requirement of RCD product standards that the resetting means must be a trip free type which ensures that the solenoid and load contacts cannot be held closed if the RCD trips in response to a residual current. The solenoid mechanism is therefore designed such that when its contact opens it remains open until manually reset. This adds to the complexity of the design of the resetting means. The design of FIG. 1 has a number of drawbacks, as follows.                The current required to close the relay RLA will be quite large, typically about 15-20 mA. This results in considerable power dissipation in R1 and RLA, which have to be suitably rated to handle this power.        The level of AC supply voltage at which RLA automatically closes and opens is highly dependent on the value and tolerance of C1 and the efficiency of RLA, resulting in variances in these values from one RCD to the next.        The solenoid SOL has to be suitably rated to handle the relatively high operating voltage and current required to ensure reliable opening of the solenoid contacts SW2.        The solenoid contacts are fitted in the high voltage part of the circuit, giving rise to problems of voltage rating and dielectric strength, etc.        SCR1 has to be suitably rated to withstand the relatively high operating voltage and current to which it is exposed.        The circuit requires a full wave bridge rectifier to ensure adequate supply current under all conditions.        
It is an object of the present invention to provide an RCD which mitigates one or more of the above disadvantages.